Adsorption on Bimetallic Alloys

Platinum-based bimetallics (Pt-M, M=Ti, Cr, V, Mn, Fe, Co, Ni, Cu, etc.) have been shown to exhibit enhanced activity toward the ORR. Several rationales have been proposed [47, 48] including enhanced chemisorption of intermediates a lattice change of Pt that results in the shortening of Pt-Pt inter-atomic distances by alloying the formation of skin Pt which has increased d-electron vacancy of the thin Pt surface layer, caused by the underlying alloy and a redox-type process involving the first-row transition alloying element and the anchor effect of alloy metals on a carbon carrier. Theoretical studies have been conducted in an effort to understand the enhanced activity of the bimetallic alloys. 

Study of oxygen adsorption on pseudomorphie PtRu overlayers [60] revealed that the properties of the PtRu overlayer systems are not intermediate between those of the two constituents. Rather, they bind adsorbates less strongly than the two elementary metals. The same observation was made in a Pd-alloy study [58]. 

Hyman and Medlin also examined intermediates adsorption on model Pt(l 11)-alloy surfaces [65]. Their study illustrated that the primary mechanism of OH destabilization on a Pt/PtsM surface is due to compressive strain, which also destabilizes adsorption of all of the intermediates. This is consistent with the study on Pd alloy [58]. Whereas shifts in binding energy due to strain correlate well for all of the intermediates examined, shifts in O adsorption energy resulting from ligand contributions were found not to correlate with the other intermediates. The authors also pointed out that the adsorption energy of oxygenate intermediates does not depend solely on the f-band center of the surface, but is also dependent on the electron density near the Fermi level. 

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